3.1.2. Human–Earth System Coupling Theory

The human–Earth system is a dynamic, open, large, and complex system composed of subsystems related to population, resources, ecology, the environment, the economy, and society. Each subsystem has a relationship of mutual influence, mutual promotion, and restrictions. Furthermore, there are frequent exchanges of personnel, materials, energy, capital, technology, and information inside and outside the system, and the complex feedback structure inside the system presents obvious non-linear and dissipative characteristics [45,46]. Human–Earth coupling refers to the dynamic correlation between humanity and nature through the interactions and complex feedback mechanisms between human economic and social activities, resources, ecology, and the environment [47,48]. The human– Earth coupling system emphasizes the multi-dimensional coupling in organization, space, and time. Through the complex interaction between one element and many elements, as well as the interaction and coupling between many elements, this system embodies the characteristics of comprehensiveness, complexity, and nonlinearity at a high level [49,50]. Optimization of the human–Earth system refers to the reasonable combination and matching of sub-systems and components of the regional human–Earth system in the space–time process [51]. The PLES system involves different resource elements and their combinations, such as water resources, land resources, and energy resources. These elements have extremely complicated mutually influential relationships. The overall optimization of PLES should take the human–Earth coupling theory as its core, and effectively measure the nonlinear effects among the subsystems and elements in the system; scientifically clarify the ordered structure of matter, energy, and information in the system; and emphasize the multi-dimensional coupling in organization, space, and time.

#### 3.1.3. System Science Theory

According to the perspective of system theory, a system refers to an agglomeration of many elements with specific functions and organic connections [52]. From this perspective, "space" is the collection of all material flow, energy flow, and information flow generated by the interaction between man and nature, including natural resource elements (water resources, land resources, energy, etc.), the artificial environment, and other material space. "Space" also includes attribute space, which is formed from changes to the distribution, structure, and function characteristics of material space, along with the formation and development of technology and information [53]. Territorial space is a large, dynamic, complex system involving the interaction of multiple factors. Territorial space is a dynamic, multi-dimensional, and complex human–Earth relationship space–time system developed along the time axis under the participation of human activities, with time–space–human is its core element [54]. According to the theory of "element–structure–function" in system theory, system structure is the foundation of system–function realization. However, the system structure depends on the organizational form and action mode of the system elements. Only by systematically splitting the territorial space structure, and analyzing the interactions between space and function, can we model comprehensive zoning, and explain the geographical decision mechanism of territorial space optimization using a quantitative decision analysis model [55].

In terms of the "territorial space" system, the research topics cover land resources, water resources, mineral resources, the ecological environment, economic and social development, and other multi-dimensional and multifaceted factors. Only the comprehensive integration and overall optimization of these spatial elements can maximize the PLE functions, and achieve the ultimate goal of the optimal allocation of territorial space [11,56–58]. Therefore, the identification of key elements is the basis of structural optimization and functional realization. As the main carrier of future spatial planning, land resources represent the core of PLES optimization. The optimization of PLES is driven and guided by optimizing the quantity ratio and spatial allocation of land.
